1. Technical Field
The invention relates to a method of manufacturing an optical information recording medium that includes a signal substrate, a signal recording layer formed on one of its principal surfaces and a transparent cover layer (for example, with a thickness of about 0.1 mm) formed on the signal recording layer, in which a focused light is transmitted through the cover layer in the process of signal recording and reproducing.
2. Description of the Related Art
Optical information recording media are known for their high-density recording and reproducing performance through a laser beam, and their use has become widespread. In recent years, for recording video information such as movies as well as sound information, digital versatile discs (DVDs) as large-capacity optical discs have been developed for market and have already become widespread. The high-density optical disc such as a DVD is used with a short wavelength laser beam and a large numerical aperture (NA) objective lens. However, such a short wavelength and a large NA reduce the tolerance of warpage (tilt) of the disc. A thinner substrate is effective at increasing the tolerance of tilt, and for example, the DVD substrate has a thickness of 0.6 mm for use with a laser beam with a wavelength of 650 nm and with an NA of 0.60. However, a single 0.6 mm thick resin substrate has low mechanical strength and tends to tilt. Therefore, two substrates are bonded to form the DVD with the information recording surface placed inside. Using the bonded structure of two substrates, commercialized is a one-sided reproducing two-layer DVD in which both of the information recording surfaces are read from the one substrate side which includes a translucent reflection layer. Such a DVD has one substrate having a translucent reflection layer of gold, silicon, or the like formed on its information recording surface, and the other substrate has a general reflection layer of aluminum or the like formed on its information recording surface, and the recording surfaces are placed inside. A similar two-layer structure is also proposed as a rewritable DVD in which the information recording layer surface has a rewritable thin film in place of the metal reflection layer.
The use of a blue violet laser beam source (with a wavelength of around 400 nm) is also proposed for higher density. In such a case, for example, the transparent cover layer on the recording/reproducing side has a thickness of about 0.1 mm, and a lens with a NA of about 0.85 is used to form a hyperfine laser spot in recording or reproducing signals. For example, the transparent cover layer is formed by one of the following two methods:
(A) a transparent substrate with a thickness of slightly less than 0.1 mm is bonded with an adhesive to the signal recording layer side of a 1.1 mm thick signal substrate; and
(B) a signal recording layer side of a 1.1 mm thick signal substrate is coated with a transparent resin layer with a thickness of about 0.1 mm.
In the first method (A), a 0.1 mm thick transparent cover layer can easily be formed by attaching the transparent substrate with a thickness of slightly less than 0.1 mm. However, such a transparent substrate with a thickness of slightly less than 0.1 mm is required to be extremely uniform in thickness and therefore expensive. In contrast, the second method (B) uses a transparent resin such as a resin for CD protection and a resin for DVD bonding and therefore can be performed at low cost.